1. Field of the Invention
The present invention relates to a position detecting device for detecting a movement position of a piston rod or the like of a hydraulic piston equipped in a fork lift or the like, and also relates to an industrial vehicle installed with the position detecting device.
2. Description of the Related Art
A fork lift called as an industrial vehicle is generally equipped with a hydraulic cylinder for moving a fork upward and downward. The hydraulic cylinder incorporates therein a piston rod as a movable member, which is linearly reciprocated by supply and discharge of working oil. The hydraulic cylinder is provided at its bottom portion with an ultrasonic wave sensor which emits an ultrasonic wave toward the lower end surface of the movable member and receives the ultrasonic wave reflected by the lower end surface of the movable member, and a position detecting device is employed to detect a position of the movable member on the basis of a time elapsed from a point at which the ultrasonic wave is emitted to a point at which the ultrasonic wave is received.
The above conventional position detecting device is designed to be constant in terms of both the strength of the ultrasonic wave emitted from the ultrasonic sensor and the sensitive level of the ultrasonic wave when it is received. In the case where the temperature of the working oil within the hydraulic cylinder is low, the high viscosity of the working oil is likely to attenuate the ultrasonic wave in the course of the propagation of the ultrasonic wave through the working oil. Further, the propagation of the ultrasonic wave is likely to be worsened at the interface between the ultrasonic wave sensor and the working oil. For these reasons, in the case where a distance between the ultrasonic wave sensor provided at the bottom portion of the hydraulic cylinder and the lower end surface of the movable member is large, an amplitude of the echo, i.e. the ultrasonic wave reaching the ultrasonic sensor by being reflected by the lower end surface of the movable member and propagated through the working oil, becomes smaller than the sensitive level as shown in FIG. 13, making it difficult to receive the ultrasonic wave. In this case, the position of the movable member cannot be detected.
In contrast, in the case where the temperature of the working oil within the hydraulic cylinder is high, the attenuation of the ultrasonic wave in the course of the propagation of the ultrasonic wave through the working oil is small because of the low viscosity of the working oil. Therefore, if the ultrasonic wave emitted from the ultrasonic wave sensor provided at the bottom portion of the hydraulic cylinder is strong, there are generated not only the echo, that is, the ultrasonic wave reaching the ultrasonic sensor by being reflected on the lower end surface of the movable member and propagated through the working oil, but also a reverberation which is propagated through a cylindrical wall of the hydraulic cylinder to reach the ultrasonic wave sensor and which has a higher level than the sensitive level of the ultrasonic wave sensor as shown in FIG. 14. In this case, since the reverberation propagated through the metal cylindrical wall of the hydraulic cylinder reaches the ultrasonic wave sensor before the echo reaches the ultrasonic wave sensor, the time elapsed from the point at which the ultrasonic wave is emitted from the ultrasonic wave sensor to the point at which the ultrasonic wave is received is inadequately determined on the basis of the reverberation, not the echo. Therefore, the position of the movable member detected on the basis of this time elapsed is inaccurate.
In the typical position detecting device, regardless of the distance between the ultrasonic wave sensor provided at the bottom portion of the hydraulic cylinder and the lower end surface of the movable member, the strength (the amplitude) of the ultrasonic wave emitted from the ultrasonic wave sensor is constant as well as the sensitive level when the ultrasonic wave is received is constant. Therefore, in the typical position detecting device, in the case where the distance between the ultrasonic wave sensor provided at the bottom portion of the hydraulic cylinder and the lower end surface of the movable member is relatively small, the reverberation propagated through the metal cylindrical wall of the hydraulic cylinder reaches the ultrasonic wave sensor before the echo reaches the ultrasonic wave sensor as shown in FIG. 15, and consequently, the time elapsed from the point at which the ultrasonic wave is emitted from the ultrasonic wave sensor to the point at which the ultrasonic wave is received is inadequately determined on the basis of the reverberation, not the echo. Therefore, the position of the movable member detected on the basis of this time elapsed is inaccurate.
On the other hand, if the distance between the ultrasonic wave sensor and the lower end surface of the movable member is relatively large, almost no reverberation is generated but the amplitude of the ultrasonic wave reflected by the lower end surface of the movable member becomes smaller than the sensitive level before the ultrasonic wave is received by the ultrasonic wave sensor as shown in FIG. 16, making it difficult to receive the ultrasonic wave. Consequently, the position of the movable member cannot be detected.
An object of the present invention is to provide a position detecting device, and an industrial vehicle having the position detecting device, which can accurately detect a position of a movable member of a hydraulic cylinder.
A position detecting device according to the present invention is a position detecting device for detecting a position of a movable member, movable within a hydraulic cylinder, using an ultrasonic wave, the device including: an ultrasonic wave sensor which emits an ultrasonic wave toward the movable member and receives the ultrasonic wave reflected by the movable member; position calculating means for counting a time elapsed from a point at which the ultrasonic wave is emitted to a point at which the ultrasonic wave is received, and calculating a position of the movable member relative to the ultrasonic wave sensor on the basis of the time elapsed thus counted; and ultrasonic wave strength setting means for variably setting a strength of the ultrasonic wave emitted from the ultrasonic wave sensor.
A position detecting device according to the present invention is a position detecting device for detecting a position of a movable member, movable within a hydraulic cylinder, using an ultrasonic wave, which may include: an ultrasonic wave sensor which emits the ultrasonic wave toward the movable member and receives the ultrasonic wave reflected by the movable member; position calculating means for counting a time elapsed from a point at which the ultrasonic wave is emitted to a point at which the ultrasonic wave is received, and calculating a position of the movable member relative to the ultrasonic wave sensor on the basis of the time elapsed thus counted; and reception sensitive level setting means for variably setting a reception sensitive level for the ultrasonic wave received by the ultrasonic wave sensor.